Debugging NGINX can be a complex task, but with the right tools and techniques, you can identify and resolve issues efficiently. This section will cover various methods and tools to help you debug NGINX effectively.

1. Error Logs: NGINX maintains error logs that provide detailed information about issues encountered.
2. Access Logs: These logs record all requests processed by NGINX, useful for tracking down issues related to client requests.
3. Debug Logging: Enabling debug logging can provide more granular details about NGINX's operations.
4. Configuration Testing: Before applying changes, testing the configuration can prevent potential issues.
5. Common Debugging Tools: Tools like curl, nginx -t, and strace can be invaluable for debugging.

NGINX error logs are the first place to look when something goes wrong. They provide detailed information about errors and warnings.

You can configure the error log level in the NGINX configuration file:

The log levels available are:

• debug
• info
• notice
• warn
• error
• crit
• alert
• emerg

This configuration will log messages at the error level and above.

Access logs record all requests processed by NGINX. They can help you understand the traffic patterns and identify issues related to client requests.

You can configure the access log format and location in the NGINX configuration file:

Debug logging provides detailed information about NGINX's operations, which can be useful for diagnosing complex issues.

To enable debug logging, you need to set the error log level to debug:

Before applying changes to the NGINX configuration, you should test it to ensure there are no syntax errors.

Use the following command to test the NGINX configuration:

curl is a command-line tool for transferring data with URLs. It can be used to test HTTP requests and responses.

This command will display the HTTP headers of the response from the server.

strace is a diagnostic, debugging, and instructional userspace utility for Linux. It is used to monitor the system calls used by a program and the signals it receives.

This command will attach strace to the NGINX process and display the system calls it makes.

1. Configure NGINX to log errors at the warn level.
2. Enable access logging with a custom log format.
3. Enable debug logging.
4. Test the NGINX configuration.
5. Use curl to make a request to the NGINX server and check the logs.

1. Configure error logging:

   error_log /var/log/nginx/error.log warn;
   

2. Configure access logging:

   log_format custom '$remote_addr - $remote_user [$time_local] "$request" '
                     '$status $body_bytes_sent "$http_referer" '
                     '"$http_user_agent" "$http_x_forwarded_for"';
   
   access_log /var/log/nginx/custom_access.log custom;
   

3. Enable debug logging:

   error_log /var/log/nginx/error.log debug;
   

4. Test the configuration:

   $ sudo nginx -t
   nginx: the configuration file /etc/nginx/nginx.conf syntax is ok
   nginx: configuration file /etc/nginx/nginx.conf test is successful
   

5. Use curl to make a request:

   curl -I http://localhost
   

   Check the logs:

   tail -f /var/log/nginx/error.log
   tail -f /var/log/nginx/custom_access.log
   

In this section, we covered various methods and tools for debugging NGINX. By understanding how to configure and interpret error and access logs, enable debug logging, test configurations, and use common debugging tools, you can effectively diagnose and resolve issues with your NGINX server. This knowledge prepares you for more advanced topics and ensures your NGINX server runs smoothly.